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Darwish" , Alexandre Chartre , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] x86/its: use Sapphire Rapids+ feature to opt out Message-ID: References: <20251017011253.2937710-1-jon@nutanix.com> <20251020194446.w2s5f6nr7i7vw4ve@desk> <20251020204026.a7xuhnagsqo4otpx@desk> <225134fd-033f-4d63-b88c-772179054694@intel.com> <20251020224159.xkhfs3phai5o6rzb@desk> <1D952EBC-CA16-49FE-8AD0-48BCE038332B@nutanix.com> <73a22a19-c492-4a75-8352-a4cfac47d812@intel.com> <883EDC14-1A26-46F4-B9E9-C75A6DF07195@nutanix.com> In-Reply-To: <883EDC14-1A26-46F4-B9E9-C75A6DF07195@nutanix.com> X-ClientProxiedBy: BY5PR20CA0034.namprd20.prod.outlook.com (2603:10b6:a03:1f4::47) To DM4PR10MB8218.namprd10.prod.outlook.com (2603:10b6:8:1cc::16) Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR10MB8218:EE_|SN7PR10MB6449:EE_ X-MS-Office365-Filtering-Correlation-Id: f1a85c42-da50-4762-1025-08de10b664a3 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014; 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charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMDE4MDAyMiBTYWx0ZWRfX5rriMrsFAAYd WdP69iZaoqbzicXrvULdEn/PmeitMqx114Pg1xk6S7paQLodr3sFuNZ4QoVg0nkYREoEcBbcNAy 32m5gRT1jUpwLIJ7T0iWxWM4ls7MRDano9RGMOInMYKBdDGQ/MZRxoHR2DSx8JmS4yhCQ3s/N9C aS80rcMqBsxlXWiSOqesjiJec7ipONJhatW/UrZNoIumHZDVRXLBnCT+0mXriO1LDB7QNi9+c0k WhNEtb+ewOU6sS0/D/84ai6nfI54xyy0NrFSfeL/6JQwBIyyhdzyEnNSIEGRDQJkNyjZfzqeicB xw3sI6R9LsV0T7gtn6l7qy2DCCyL2yDkaD11xet/xeX4Jph1FCYuwZQnT/5e3DfGYH7RVZvYnqc y/vVJEBzNeKpLRuuuhRYl/lZONWVOS0AY7UG2Vi7qb9jpMUpyXw= X-Proofpoint-ORIG-GUID: iPhrKOPws-2xKZDmFxwZ_ZcfHY5gSJ3f X-Proofpoint-GUID: iPhrKOPws-2xKZDmFxwZ_ZcfHY5gSJ3f X-Authority-Analysis: v=2.4 cv=FuwIPmrq c=1 sm=1 tr=0 ts=68f7a698 b=1 cx=c_pps a=e1sVV491RgrpLwSTMOnk8w==:117 a=e1sVV491RgrpLwSTMOnk8w==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=x6icFKpwvdMA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=yPCof4ZbAAAA:8 a=QyXUC8HyAAAA:8 a=s2Io-HG2iaxk0UaDc0sA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 cc=ntf awl=host:13624 On Tue, Oct 21, 2025 at 02:39:15PM +0000, Jon Kohler wrote: >=20 >=20 > > On Oct 21, 2025, at 10:01=E2=80=AFAM, Dave Hansen wrote: > >=20 > > > >=20 > > On 10/21/25 06:40, Jon Kohler wrote: > >> So to simplify it down: > >> A guest VM that updates to a ITS-enabled guest kernel sees performance > >> impacts on non-vulnerable hardware, when running on non-BHI_CTRL and/or > >> non-ITS_NO hypervisors, which is a very easy situation to get into, es= pecially > >> on QEMU with live migration-enabled pools. > >=20 > > By non-$FEATURE, do you mean that they chose to not enumerate those > > features, or that they are completely ignorant of them? >=20 > Both cases are true for QEMU. >=20 > For ITS_NO, it is an allowed feature, but its not part of a QEMU model by > default, so the higher level control plane whatever that may be would nee= d to > specifically turn it on, its not automatic. >=20 > For BHI_CTRL, depending on what QEMU the VM was originally *started* on, > the guest may have access to Sapphire Rapids models, but BHI_CTRL may > not have existed in the QEMU source at that time, as those were introduced > into two different timeframes. QEMU provides now a mechanism to update itself to a newer version. See https://blogs.oracle.com/linux/post/qemu-live-update That should solve your QEMU problem.